When an on-off operation of a semiconductor switching element, i.e., a switching of the semiconductor switching element is conducted in a power conversion device (for example, a three-phase inverter), a switching loss is caused. In a case that this switching loss is large, is there is a problem such as an efficiency decrease of the power conversion device and an upsizing of the power conversion device because of the necessity of a cooling device for removing a generated heat. Contrary to this, in a case that a switching frequency is lowered in order to reduce the switching loss, a waveform-control performance is reduced due to a decrease of the number of switching operations. That is, a rate of higher harmonics relative to a fundamental wavelength is increased.
As a countermeasure, a two-arm modulation method in which the switching frequency is lowered while suppressing the reduction of waveform-control performance to the minimum is known. In this two-arm modulation method, voltage command values of three phases are corrected to cause one of the voltage command values of the three phases to become greater than or equal to an amplitude value of a triangular-wave carrier. Thereby, a switching of the semiconductor switching element of the one phase is stopped for a certain period, so that an average switching frequency of the three phases is lowered. (see Patent Literature 1)
FIG. 13 is a time chart of voltage command values of three phases and a triangular-wave carrier in the two-arm modulation method. FIG. 14 (a) is a time chart of voltage command values of three phases in a three-arm modulation method. FIG. 14 (b) is a time chart of a correction amount (compensation amount) α which is used in the two-arm modulation method. FIG. 14 (c) is a time chart of voltage command values in the two-arm modulation method. A modulation factor (modulation percentage) of the voltage command value of each phase is denoted by “m”.
As shown in FIG. 14 (a), among the voltage command values V*U, V*V and V*W of three phases, for example, the voltage command value V*U takes a maximum level in an interval (zone) “A”, and the voltage command value V*W takes a minimum level in an interval “B”. In order to suspend the switching of the semiconductor switching element for one phase in these intervals “A” and “B”, the correction amount α (waveform of FIG. 14 (b)) is added to each of the voltage command values V*U, V*V and V*W of three phases. This correction amount α is calculated by subtracting the voltage command value V*U from 1 (i.e., 1−V*U) in the interval “A”, and by subtracting the voltage command value V*W from −1 (i.e., −1−V*W) in the interval “B”. As a result, as shown by waveforms of V*U+α, V*V+α, and V*W+α in FIG. 14 (c), the voltage command value of one of the three phases always exceeds or becomes equal to the amplitude value of triangular-wave carrier with a phase change done every interval of 60°. Thereby, the semiconductor switching element of the one phase exceeding or equal to the amplitude value of triangular-wave carrier stops (suspends) its switching operation. Thus, by adding the correction amount α to the voltage command values V*U, V*V and V*W of three phases, the voltage command values V*U+α, V*V+α, and V*W+α for the two-arm modulation method can be produced.
Moreover, Patent Literature 2 discloses a motor control apparatus that performs a changeover between the two-arm modulation method and the three-arm modulation method.